Uncheck the Firefox Default browser in
Tools > Options > Advance > General > System Default > Always check to see if firefox is the default browser on Start up
Check the option in Waterfox
Tools > Options > Advance > General > System Default > Always check to see if firefox is the default browser on Start up
https://support.mozilla.org/en-US/kb/how-make-web-links-open-firefox-default?esab=a&s=firefox+default+browser&r=0&as=s#os=win7&browser=fx17

Helpful Reply

You may need to check the related registry keys in HKEY_CLASSES_ROOT to verify that they link to Waterfox.

Also make sure that Firefox isn't running in case the Waterfox startup code detects an open Firefox instance and uses that.

You may need to check the related registry keys in HKEY_CLASSES_ROOT to verify that they link to Waterfox.
Also make sure that Firefox isn't running in case the Waterfox startup code detects an open Firefox instance and uses that.